Begin by lightly greasing a 9 x 9-inch baking pan and lining it with parchment paper.
Chop the Mars bars into small pieces.
In a medium sized saucepan, over low heat, melt the 2/3 cup butter.
Add the Mars bars and stir constantly until the candy bar pieces are fully melted and smoothly incorporated into the butter.
Remove from heat and quickly stir in the Rice Krispies cereal.
Press evenly into the bottom of the prepared pan and chill for about an hour before adding the chocolate top.
To prepare the chocolate top:
Melt the 1/3 cup of butter, over low heat, in a double boiler.
Then add the chocolate chips, see note below, and stir constantly until the chocolate is completely melted.
Pour evenly over the cookie base and let set until firm, before cutting into squares or bars.
Notes
I use milk chocolate chips for the topping on these cookie bars; however, if you want to use semi-sweet or dark chocolate chips, that's perfectly fine too.